You can't put MX records in a hosts file, and since a DNS server usually returns an A record when it's asked for an MX record, putting the A records in your hosts file won't work either. The /etc/hosts file will never be consulted, but that's not Kerio's fault. It's just the way (most) resolvers work.

Also, as long as you aren't the authoritative DNS for their domain, adding their domain to you own DNS won't work either. If something like that had worked, the internet as we know it would cease to exist.

You could, theoretically, set up your own caching name server and play some tricks, OR, trick your DNS into believing it is the authoritative NS for their domain, but both "solutions" are well beyond the scope of this forum (and stupid to boot).

OR, you could set up a gateway/relay for your outgoing mail (e.g. use /etc/staticroutes with Exim) and force it to route all the destination domain's mail to a specific IP.

But then again, it's probably less work getting the other guy to cooperate. If you can't get to him, move up the ladder.

On the other hand, building your own smarthost will get you all the hot chicks

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